11142004, 06:39 PM  #1 (permalink) 
Crazy
Location: College Station, TX

Dumb Probability Question
The question is not dumb, i just don't like math.
A box contains two defective Christmas tree lights that have inadvertently mixed with eight nondeffective lights. IF the lights are selected one at a time without replacement and tested until both defective lights are fund, what is the probability that both defective lights will be found after three trials. I think that you work it out like this to find the answer, but i am not sure: (2/10)*(2/9)*(1/8)=(1/180) Thanks
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11152004, 03:36 AM  #5 (permalink) 
Psycho
Location: on my spinning computer chair

first try
you get defective/nondefective/defective (2/10) (8/9) (1/8) = 1/45 second try nondefective/defective/defective (8/10) (2/9) (1/8) = 1/45 probability = 1/45 + 1/45 = 2/45 there couldn't be defective/defective/nondefective because according to you, it must be exactly 3 trials, so why would you want to try again after you get 2 defectives. however.. if you want to consider defective/defective/nondefective then it should be 2/45 + (2/10) (1/9) (8/8) = 1/15 Hope this helps. 
Tags 
dumb, probability, question 
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